A network switch is a device that communicatively connects two or more physical links in a network. A network switch can translate physical signals input on one type of link for output on another type of link. The network switch may also perform other functions such as determining the output link(s) to which a particular input should be translated. Exemplary network switches include public switched telephone network (PSTN) switches, Ethernet switches, and asynchronous transfer mode (ATM) switches, among others.
In a network switch that translates between two or more different types of physical links, the physical translation of an input signal to an output link may give rise to a partial reflection of the input signal back toward the source of that signal, in addition to the intended transmission of the signal on the output link. For example, the circuitry that performs the translation may suffer an impedance mismatch between the different types of links. Such reflected or leaked signal is commonly referred to as “echo” because the source of the input signal receives a distorted version of the signal that is echoed back from the switch, the echoed signal being delayed by the round-trip transmission time between the source and the switch. This may also occur internally to the source where the signals being transmitted are echoed back to its own receiver. The echoed signal may then combine with any signal transmissions intend for the source, thereby distorting or polluting those transmissions. The portion of the translation path in the switch on which echo is generated is called the echo path. The echoed signal is one of the biggest sources of interference in 10GBASE-T, especially when the link-partner is located at distances comparable to 100 meters.